The present invention relates to a technique for recording information on a record medium along spiral or concentric information tracks in an optically readable form and for reproducing the information thus recorded.
In an instance of effecting an optical recording and reproducing while using a disc-shaped record medium, it is necessary to avoid interference between adjacent tracks caused by vibration due to a deviation in movement of an optical system for projecting a recording or reproducing light beam onto the record disc and a small eccentricity of a rotating center which might be produced upon exchanging the record discs with respect to a turntable.
FIG. 1 is a schematic view illustrating a general construction of a known optical recording and/or reproducing apparatus. In one instance of recording an information signal, a semiconductor laser light source 1 is driven by a laser driver 2 to generate a light beam having an intensity modulated with the information signal to be recorded. The light beam thus generated is converted by a collimator lens 3 into a parallel light flux which is then focussed by an objective lens 4 onto a record medium 5 as a small light spot to form a concentric or spiral track 10 which consists of an array of pits corresponding to the information signal and having a property which can be read out optically.
During reproduction, the light beam having a constant intensity is generated from the semiconductor laser light source 1 and is focussed by means of the collimator lens 3 and the objective lens 4 onto the information track 10 as a light spot, while the record medium 5 is rotated. The reflected light from the record medium 5 is modulated in accordance with the recorded information signal and is collected by the objective lens 4. The light emanating from the lens 4 is transmitted through a quarter-wavelength plate 8 twice and is then reflected by a polarizing beam splitter 25 toward a photoelectric converting device 6. An output signal from the photoelectric converting device 6 is amplified by an amplifier 7 to produce a reproduced information signal.
During the recording, the focussed light spot is shifted at a constant speed in a radial direction with respect to the disc-shaped record medium 5 by moving an optical system 9 including the optical elements 1, 3, 4, 8 and 25 in the radial direction as shown by a double-headed arrow in FIG. 1. During this movement of the optical system 9, if the travelling speed of the system 9 fluctuates, pitches of successively formed tracks might be varied. Such a variation in the track pitch might also be produced due to mechanical vibration of a turntable and eccentricity of the record medium. In an extreme case, adjacent tracks might be partially superimposed or crossed with each other and the information in the superimposed or crossed portion could not be effectively reproduced. Further, the eccentricity is usually produced whenever the record medium 5 on the turntable is exchanged with another record medium. During reproduction, the above mentioned problem also occurs. Therefore, it is necessary to space the successive tracks by a sufficiently large pitch so as not to interfere with each other. This causes a serious drawback in that a high density recording (which is one of the most important properties of the record medium of this type) could not be attained.
In order to avoid the above mentioned drawback, a so-called tracking control is usually effected so that the recording or reproducing light beam can follow precisely the information track. For instance, in IEEE spectrum August 1979, pages 26 to 33, there has been proposed a pregrooved disc having grooves previously formed therein. During the recording, the grooves are reproduced optically to derive a tracking error signal and the position of the recording light beam is controlled by the tracking error signal. In this instance, since the information tracks are always formed along the grooves, undesired interference between the tracks can be eliminated. However, in such a pregrooved disc, the information pits are formed in the grooves and thus, there is a possibility that the information might be deteriorated by defects inherently formed in the grooves. Such defects might not cause a serious problem for analog information such as video and audio signals, but for digital data such defects might cause impermissible errors.